1. Field of the Invention
The present invention relates to the design of an input stage to an amplifier or comparator; in particular, the present invention relates to the design of a cascode amplifier in an input stage of an amplifier or a comparator.
2. Discussion of the Related Art
The folded cascode amplifier, also called a common-source common-gate amplifier, is known to provide high frequency, high gain and high bandwidth performance. The design and operational characteristics of folded cascode amplifier are discussed extensively in (i) CMOS Analog Circuit Design, by P. Allen and D. Holberg, pp. 421-426, published by Harcourt Brace Jovanovich College Publishers (1987), (ii) "MOS Operational Amplifier Design--A Tutorial Overview" by P. Gray and R. Myer, IEEE Journal of Solid State Circuits, Vol. SC-17, No-6, December 1983, pp. 969-982, (iii) "Design Techniques for Cascoded CMOS Op Amps with Improved PSRR and Common-mode Input Range", by D. Ribner and M. Copeland, IEEE Journal of Solid State Circuits, Vol. SC-19, No. 6, December 1984, pp. 919-925; and (iv) "High Frequency CMOS Switched Capacitor Filters for Communication Applications" by T. Choi et al, IEEE Journal of Solid State Circuits, Vol. SC-18, No. 6, December 1983, pp. 652-64.
FIG. 6 shows a conventional folded cascode input stage amplifier 100. As shown in FIG. 6, the gate terminals 151 and 150 of PMOS transistors 104 and 105 provide the input terminals for receiving a differential input voltage intola first stage 100a of folded cascode amplifier 100. A current source 101 provides a current I, which is split between transistors 104 and 105. The differential input voltage at the gate terminals of transistors 104 and 105 is converted in a second stage 100b into a unipolar output voltage at terminal 110. Second stage 100b includes PMOS transistors 106 and 107, NMOS transistors 108 and 109, and current sources 102 and 103 connected in a cascode configuration. NMOS transistors 108 and 109 receive a common bias voltage V.sub.BIAS to perform a level shifting function for transistors 106 and 107.
During operation, when a differential input voltage appears across input terminals 150 and 151, the current in transistor 104 decreases. As a result, the current in both transistors 106 and 108 increases. The current in transistor 106 is mirrored to transistor 107, so that the current in transistor 107 also increases. At the same time, the current in transistor 105 increases, resulting in a decrease in current in transistor 109. The net result of the increase in current in transistor 107 and the decrease in current in transistor 109 is a net .increase in output current at the output node 110. With a high impedance load at output node 110, the voltage at node 110 rises towards the supply voltage V.sub.cc until equilibrium occurs. Conversely, if a negative differential voltage is impressed across terminals 150 and 151, the current in transistor 109 increases and the current in transistor 107 decreases, so that the voltage at terminal 110 falls towards ground voltage. Thus, the voltage at terminal 110 follows the differential voltage across terminals 150 and 151.
At equilibrium, transistors 104 and 105 each carry a current I/2. However, the differential voltage across terminals 150 and 151 can be large enough so that virtually all of the current in current source 101 flows through transistor 104. If the current at current source 102 is less than I, then transistors 106 and 108 conduct no current. Further, the current in transistor 105 is limited to I, so that transistor 105 is forced to operate in the "triode" region, which has very low transconductance. As a result, the transconductance and gain of circuit 100 are severely impaired. In other words, the effective current provided at node 110 is less than I. To prevent this result, an additional "overdrive" current I.sub.od is provided. Thus, the current sources 102 and 103 each sink a current equal to I+I.sub.od, to yield a total quiescent current in folded cascode amplifier 100 of 2*I+2*I.sub.od.
One major disadvantage of a folded cascode amplifier is the large quiescent currents, leading to high power dissipation, which is not desirable in many applications requiring low power operation.